A classic study-case in user interface design awaits you at you local Wal-Mart (or any other place that offers self-checkout).
eWEEK.com's Special Report on Self-Checkout Technology Self-scanning tests at Wal-Mart, supermarkets register approval PFFT: Stuff your Self Checkout Wal-Mart, Albuquerque, NM - self checkout fast lane stressful and not consumer friendly
Personally I like the self-checkout (when it is open) as it saves time and I get out the door faster. On the other hand software and user interface is a good part of my life's work, so I can figure out how the designer meant for things to work. Regular folks tend to be more easily mystified.
Tonight was a classic example. A mother was at the self-checkout with her two young daughters. The older daughter was handing items to her mother to scan, and was clearly less intimidated by anything with a screen. The mother would scan the item, place it in the bag, and the younger daughter would take the bags and place them in the cart.
All perfectly reasonable.
But when a bag was removed the screen would present a complaint. Now though I knew why the complaint appeared (the software was counting on the ever-increasing weight of the bag to monitor the checkout process), even I found the message presented somewhat obscure. The mother was plainly puzzled by the opaque message. The older daughter had learned that when computers present obscure messages, the best response is to punch buttons until the message goes away.
In the end the daughter's stategy worked. At points the software threw up full-screen messages with the demand that we should all wait for a clerk to appear. The nearby clerks did not seem in any way interested. The older daughter knew better and simply kept punching buttons, the mother kept scanning items, and the younger daughter kept loading bags into the cart.
Now I have no idea if the final total was accurate, but in the end they paid and left. An older male standing behind me was plainly trying to make sense of all that was going on, and was just as plainly puzzled. He turned to his daughter (or grand-daughter?) and muttered something about "labor-saving" devices and Wal-Mart saving the price of a clerk.
From the above we can see pretty clearly why ActiveX controls were an enormous security problem in Internet Explorer :).